The invention relates generally to multiple compressor refrigeration systems, and more particularly to improvements in time delay controls for the refrigeration system.
In recent years many advances have been made in the refrigeration art and especially in the commercial refrigeration field, which includes supermarket refrigeration and like installations having heavy refrigeration requirements over a wide range of temperatures from about -40.degree. F. to about 50.degree. F. (-40.degree. C. to about 10.degree. C.). So-called central refrigeration systems of the heavy multiplexing type utilize several compressors (typically either two or four) connected for parallel operation to effect refrigerant flow to and from the evaporators of a large number of refrigerated fixtures. Multiple compressor systems are generally controlled by pressure sensitive switches responsive to the suction pressure at the compressors intake so that as the suction pressure fluctuates in response to increases or decreases in system loads, the compressors will cycle on and off to maintain the common suction pressure on the system within prescribed limits to maintain proper temperature control of the refrigerated fixtures.
Fluctuation of the suction pressure is influenced by various internal (system) factors including temperature controls, defrosting apparatus and the like, and by several external factors including product loading of refrigerated fixtures, ambient temperatures and the like, and at times sudden transient increases in suction pressure may cause one or more idle compressors to start thereby rapidly reducing the suction pressure to the point where such compressors will cycle off again. Since these suction pressure changes are frequently transient in nature, the capacity of the operating compressors in the system would often be adequate to restore a normal suction pressure before there is any significant influence on the temperatures of the refrigerating fixtures. However, if the thermal load change causing the suction pressure (temperature) rise is of long duration, then the operation of one or more additional compressors may be necessary to maintain normal refrigerating fixture temperatures.
Wide fluctuations of the compressor head pressure may also be influenced by the same types of internal and external factors, and transient decreases in head pressure may be caused, for instance, by initial reduction of refrigerant loads as when a defrost cycle is started on selected fixture evaporators. Regulation of the effective condensing capacity can be controlled by the sequential and cyclical operation of a series of "zone" condenser fans controlled by pressure sensitive on-off switches so that, as the head pressure fluctuates, the fans will cycle on and off to change the condenser capacity toward maintenance of the head pressure on the system within prescribed limits. If such head pressure changes are transitory in nature, it may be desirable to maintain the operation of the condenser fans unless the decrease in head pressure persists.
It is apparent that electric power consumption will be reduced in the overall operation of the refrigeration system if additional compressors are not started in response to transient load increases, or if other system components are left idle during transient pressure fluctuations. It will also be apparent that the efficiency of such compressors will be increased when operated at lower head pressures, thereby effecting additional power savings. In the past, electric time delay relays have been used for delaying the start of additional compressors sensing an increase in suction pressure or for effecting sequential operation of other system components, but such electric relays are insensitive to the actual magnitude of the refrigerant pressure at the critical or predetermined location within the system, whereby the compressor, fan or other component controlled thereby will start no matter how small the difference is between actual pressure and the pressure switch setting. In short, heretofore there has been no simple, positive acting, pressure sensitive, time delay system for effectively obviating on/off component cycling due to sudden temporary or transient pressure changes.